Last May, contaminated fenugreek seeds caused an unusually severe outbreak of food poisoning in Germany, affecting nearly 4,000 people. 50 of those people died.
The cause was a strain of E. coli bacterium that releases a tiny protein called Shiga toxin into the blood, causing damage to blood vessels and sometimes fatal kidney failure. Worldwide, one million people die annually from infection with Shiga producing E. coli, predominantly in the developing world. Unfortunately, our best weapons against bacterial infections, antibiotics, can’t be deployed against this disease, since when the dying bacterial cells burst they release even more of the dangerous toxin.
However, a surprisingly simple and low-cost treatment strategy has been suggested by the research of cell biologists at CMU. The breakthrough came from studies of a biological process that Shiga toxin hijacks when it sneaks into our cells.
Most E. coli do not produce Shiga toxin and are entirely harmless. In fact, all mammals, including people, host a large number of harmless E. coli cells in their intestines. A few strains cause disease–normally manifesting as a treatable diarrhea–but a few of these strains are much more dangerous because they are infected with the virus that carries the Shiga toxin gene. These infected bacteria use the virus gene to make Shiga toxin, which enters the infected person’s blood stream and then breaks into the cells that line their blood vessel walls, among others.
Shiga toxin enters these cells undercover, by hitchhiking on a receptor that studs the surface of the cells, particularly those in the kidney. This receptor is periodically sucked into the cell, traveling in tiny membrane sacs to be destroyed in a waste dump/recycling center called the lysosome. The Shiga toxin avoids destruction in the lysosome by switching rides from the surface receptors to a new vehicle, called GPP130, that escorts proteins in the membrane sacs to another membrane compartment, the Golgi. Shiga toxin uses GPP130 to reach a safer location from which it can cross the internal membrane into the cell’s main compartment. From there, Shiga toxin is able to kill the cell by blocking new protein synthesis.
Professor Adam Linstedt, in the Department of Biological Sciences at CMU, didn’t set out to address the problem of Shiga toxin. He is a basic scientist studying membrane compartments within cells. Back in 1997 he discovered GPP130, the protein that helps Shiga toxin escape destruction in the lysosome, though they didn’t know the connection with Shiga until some years later. The study of Gpp130 has been important in helping us to understand how certain proteins travel to and maintain their correct location within the cell, but four years ago Linstedt was tipped off to a potentially useful quirk of GPP130. Don Smith, a toxicologist at UC Santa Cruz told Linstedt that GPP130 seemed to be subject to very precise regulation in response to the metal manganese.
Somshuvra Mukhopadhyay, a postdoctoral scientist working in Linstedt’s lab, showed that manganese causes GPP130 to be destroyed by being diverted to the lysosome. Linstedt and Mukhopadhyay have a hypothesis that this could be a way to respond to manganese levels in the cell by regulating manganese transporting proteins that also rely on GPP130 for their travels through the cell. Of course, it also suggested a simple way to render Shiga toxin harmless: if GPP130 is diverted to the lysosome by manganese treatment, maybe the hitchhiking Shiga toxin would also be destroyed.
Mukhopadhyay tested this idea using cells grown in the lab, and found that cells treated with manganese were thousands of times more resistant to Shiga toxin. Next, they tried the same trick with mice. Normally, mice injected with Shiga toxin die within four days. But those pre-treated with manganese didn’t even become sick.
This is a very exciting result, but there is still some work to go before we find out if manganese treatment could help the 150 million people affected by Shiga toxin every year. The next step will be more mice tests, but this time using infection with E. coli that produce Shiga toxin, instead of direct injection of purified Shiga toxin. They will also need to develop manganese treatment regimes that don’t require pre-treatment, since we need to be able to start treatment once symptoms of bacterial infection have already begun. But there is a good chance that manganese treatment will prove effective and safe enough to warrant clinical trials.
The great thing about this discovery is that manganese is an abundant, naturally-occuring element that has already been thoroughly tested for safety. Although high doses of manganese are toxic, and can cause neurological disease, the doses that were effective in the mice studies were not high enough to have this effect. The fact that manganese treatment is cheap is particularly appealing, given that most of the people that suffer the painful effects of Shiga toxin live in developing nations.